There has been used, for driving metal tubes into the ground, percussive air tools of relatively low driving power, and capable of driving tubes of rather small diameter.
An increase in the driving power of such percussive air tools has been until now hampered by the fact that traditionally accepted construction of their housings the results in a less technologically efficient and more labour consuming manufacture and an increase in the dimensions thereof.
A percussive air tool is known (cf., e.g., USSR Inventor's Certificate No. 236,350; IPC E21 B 7/00) for driving into the ground metal tubes serving as casings of underground communication lines and sinking piles.
This percussive air tool comprises a housing accommodating in the interior thereof a reciprocating hammer having an end chamber, and an air distributing valve means adapted to be received by the end chamber of the hammer and secured in the rear end portion of the housing. Jammed in the forward end portion of the housing by means of a conical connection is a headpiece hermetically closing the interior of the housing and subjected to the percussions of the hammer.
Prior to tube sinking the forward end portion of the housing is rigidly connected to the tube to be driven. Impact of the hammer is transmitted to the tube via the headpiece and the housing. Under the action of the impact the tube and the percussive air tool are sunk into the ground. The friction force arising between the tube and the ground prevents the tube from moving in the opposite direction due to reaction or recoil of the air tool.
A disadvantage of this percussive air tool resides the complications encountered during the manufacture of its housing, which is shaped as a thin-wall cylinder having a massive thick-wall forward end portion intended for mounting the headpiece.
This disadvantage becomes even more pronounced with an increase in the power of the air tool and consequently the diameter of its housing.
The thickened housing wall in the portion thereof adjoining the headpiece is absolutely essential because the conical connection of the housing with the headpiece is subject to considerable radial loads during operation which tend to rupture the housing.
Also known is a percussive air tool (cf. W.German Pat. No. 2,157,259; IPC E21 B 7/00, published 1973) wherein the abovementioned disadvantage is obviated. This tool serves for sinking wells and is comprised of a cylindrical hollow housing, a hammer reciprocating inside the housing and having an end face chamber, an air distributing valve secured in the rear end portion of the housing and adapted to be received by the end face chamber of the hammer, and a headpiece disposed in the forward end portion of the housing and subjected to impacts of the hammer.
The housing of this percussive air tool is fashioned as a continuous cylinder and is connected with a head portion by means of a threaded connection.
The head portion accommodates a headpiece or anvil capable of a certain amount of axial displacement, whereas the outer side of the head portion may hold a replaceable tool, such as a well expander.
The housing and the head portion of the percussive air tool are simple to manufacture.
A disadvantage inherent in the above construction resides in the low dependability of the threaded connection, which tends to loosen or even be destroyed due to the formation and growth of free play between the threads of such a connection.
The above disadvantage is further aggravated when driving tubes horizontally or obliquely and additional loads are transmitted to the threaded connection.
In addition, such a threaded connection must be manufactured to close thereby resulting in higher production costs involved.
The foregoing advantage become more tangible in air tools of higher driving power, that is devices featuring larger housing diameter.